Titanium dioxide is widely used as a catalyst and/or a catalyst support for many applications, including oxidation chemistry, hydrotreating, Claus reactions, photocatalysis, total oxidation of volatile organic compounds, and DeNOx reactions. The use of titanium dioxide as a catalyst support for the selective catalytic reduction of NOx is taught in, for example, U.S. Pat. Nos. 4,929,586, and 5,137,855. Although any crystalline form of titanium dioxide (anatase, rutile, and brookite) may be useful for catalyst applications, anatase is typically preferred, see, e.g., U.S. Pat. Nos. 5,330,953 and 6,576,589.
Unfortunately, titanium dioxide is thermally unstable when employed in high temperatures applications, such as DeNOx. At high temperatures, the titanium dioxide nanoparticles tend to coalesce, which reduces their surface area and porosity. Moreover, anatase may be at least partially converted to the less favorable rutile form at high temperature.
A number of strategies have been employed to solve these problems. One approach has been to add a second metal oxide. For example, U.S. Pat. No. 5,021,392 discloses a binary oxide support (titanium dioxide-zirconia) that is formed from the coprecipitation of salts of titanium and zirconium to form hydrosol that is aged to produce the binary oxide support. U.S. Pat. No. 5,922,294 teaches a method of making a mixed-oxide by co-hydrolysis of a mixture of the alkoxides of titanium and alumina. U.S. Pub. Appl. No. 2003/0103889 discloses a method to make a titanium dioxide-silica composite that is prepared by combining the titanium dioxide with a silica sol. When the second metal oxide is incorporated into the titanium dioxide lattice to form a homogenous single mixed oxide, the crystal lattice and the catalytic properties of titanium dioxide are typically affected.
Another approach to solve the thermal instability problem is the application of a coating to the titanium dioxide. For example, U.S. Pat. No. 5,330,953 discloses forming two coatings on titanium dioxide particles that includes a first coating comprising oxides of aluminum, silicon, zirconium and lanthanum and a second phosphate coating. In addition, U.S. Pat. No. 5,652,192 discloses a hydrothermal method of making a titanium dioxide nanoparticle coated with sulfate. The method employs hydrothermal treatment of a mixture of precursors of titanium dioxide and sulfate to make titanium dioxide nanoparticles coated with sulfate in crystal form. One problem with this approach is that the coating can affect the catalytic properties of titanium dioxide.
In sum, a new titanium dioxide nanocomposite particle and processes for making them are needed. Particularly valuable nanocomposite particles would have improved thermal stability for catalytic applications.